1. Field of Inventions
The present inventions relate generally to fuel cells and, more specifically, to the management of condensation and reaction product within fuel cells.
2. Description of the Related Art
A fuel cell converts fuel and oxidant (collectively xe2x80x9creactantsxe2x80x9d) into electricity and a reaction product. Many fuel cells employ hydrogen as the fuel and oxygen as the oxidant. Here, the reaction product is water. One such fuel cell is the proton exchange membrane (PEM) fuel cell. Each individual cell in a PEM fuel cell includes an anode and a cathode separated by a thin, ionically conducting membrane, which together are often referred to as a membrane electrode assembly (MEA). The anode and cathode, on opposing faces of the ionically conducting membrane, are comprised of a thin catalyst containing film and a gas diffusion layer. Hydrogen is supplied to the anode and oxygen supplied to the cathode. The gas diffusion layer insures that hydrogen is effectively transported to the anode catalyst and that oxygen is effectively transported to the cathode catalyst. The hydrogen is electrochemically oxidized at the anode catalyst, thereby producing protons that migrate across the conducting membrane and react with the oxygen at the cathode catalyst to produce water. The individual MEAs are stacked in electrical series with impermeable electrically conductive bipolar plates therebetween that conduct current between the anode of one MEA and the cathode of the adjacent MEA. The bipolar plates have channels formed on one side for transporting fuel over one MEA and channels formed on the other side for transporting oxidant over an adjacent MEA. The reactants, such as hydrogen and oxygen, are pumped through the channels from respective inlet manifolds to respective outlet manifolds.
Fuel cells are considered an attractive energy source for a variety of reasons. As compared to batteries, fuel cells are advantageous in that they can maintain a specific power output as long as fuel is continuously supplied and are not hampered by a charge/discharge cycle. Fuel cells are also relatively small and lightweight and produce virtually no environmental emissions. PEM fuel cells are particularly advantageous because they have relatively low operating temperatures and employ a non-liquid, non-corrosive electrolyte.
Despite these advantages, conventional fuel cells are susceptible to improvement. For example, reaction products such as water can accumulate within the channels and block reactant flow. Humidity in the reactants can also condense and accumulate within the channels. Conventional fuel cells seek to clear reaction products and condensed humidity from the channels by creating a pressure differential (or drop) between the inlet manifolds and the outlet manifolds. A desirable pressure differential is one that is sufficiently large to prevent reaction products and/or condensate from accumulating in one or more of the channels of the bipolar plate. The requisite pressure drop, which depends on a number of factors including fuel cell operating conditions (i.e. flow rate and temperature), the material and construction of the bipolar plate channels, and channel geometry, is typically between a few inches of water and 15 PSI.
Pressure is reduced in conventional fuel cells by the effects of wall friction as the reactants move through the channels. More specifically, the conventional method of creating a sufficient wall friction-based pressure differential is to make the reactant channels in the bipolar plates long and tortuous or of a small hydraulic diameter. Alternatively, the reactant flow rates can be increased in order to create greater frictional losses and pressure drops.
The inventor herein has determined that the long, tortuous reactant flow channel method of creating a pressure differential is less than optimal. For example, although it is important that the pressure differential be uniform from channel to channel to insure uniform reactant flow, it is difficult and expensive to create a series of long, tortuous channels of equal length. There are also instances where the use of long, tortuous channels is either impracticable or impossible. For example, hexagonal bipolar plates often include z-shaped flow channels which are not particularly long or tortuous. The geometry of a hexagonal bipolar plate requires the long, tortuous channels to be too far apart to achieve acceptable diffusion of the reactants into the gas diffusion electrode. As such, it is difficult to obtain the requisite pressure differential using conventional long, tortuous channels. In addition, recent advances in bipolar plate technology have resulted in relatively straight reactant flow channels. One such bipolar plate is disclosed in concurrently filed commonly assigned application Ser. No. 09/375,072, now U.S. Pat. No. 6,322, 919, entitled xe2x80x9cFuel Cell and Bipolar Plate For Use With Same,xe2x80x9d which is incorporated herein by reference.
The inventor herein has also determined that creating a pressure differential through the use of reactant channels with a small hydraulic diameter is less than optimal. The use of small hydraulic diameter reactant channels requires very tight manufacturing tolerances because without the tight tolerances friction can vary from channel to channel, which results in non-uniform reactant flow. Accordingly, although bipolar plates having small hydraulic diameters are available, their manufacture requires the use of relatively laborious and expensive manufacturing processes.
The inventor herein has also determined that increasing reactant flow rates is a less than optimal method of creating pressure differentials. Increasing the fuel flow rate results in wasted fuel, thereby reducing the efficiency of the fuel cell. Increasing the oxidant flow rate further reduces the efficiency of the fuel cell because of the additional power that is required by the associated compressor or fan.
Accordingly, one object of the present inventions is to provide a fuel cell that is capable of clearing reaction products and condensed humidity from the reactant channels. Another object of the present invention is to provide a bipolar plate assembly that creates a sufficient pressure differential between the inlet and outlet manifolds to clear reaction products and condensed humidity from the reactant channels without resorting to long, tortuous channels. Still another object of the present invention is to provide a bipolar plate assembly that creates a sufficient pressure drop between the inlet and outlet manifolds to clear reaction products and condensed humidity from reactant channels without resorting to small hydraulic diameter channels. Yet another object of the present invention is to provide a bipolar plate assembly that creates a uniform pressure differential from channel to channel and plate to plate.
In order to accomplish some of these and other objectives, a bipolar plate assembly in accordance with a preferred embodiment of a present invention includes a plurality of reactant channels defining respective inlets and outlets, the inlets of adjacent channels being adjacent one another and the outlets of adjacent channels being adjacent one another, and at least two flow restrictors respectively associated with at least two adjacent reactant channels. In one implementation, the inlets are associated with a common inlet manifold and the outlets are associated with a common outlet manifold.
The present inventions provide a number of advantages over conventional bipolar plates and fuel cells. For example, the flow restrictors create a pressure drop sufficient to clear reaction product and condensed humidity from the channels, thereby eliminating the need for the long, tortuous channels, channels of small hydraulic diameter, and excessive flow rates that create the pressure drop in conventional fuels cells. It is also relatively easy to fabricate uniformly sized flow restrictors, which results in uniform pressure differentials and uniform reactant flow through the channels without the difficulty and expense associated with the creation of channels of identical length with tight tolerances.
In those implementations of the present inventions where the inlets and outlets are associated with common inlet and outlet manifolds, the pressure differential will be determined by the flow rate and geometry of the restrictors. Should one channel become blocked, the pressure differential across the manifolds will be substantially unchanged and the pressure drop across the flow restrictor associated with the blocked channel will be zero because there is no flow. Consequently, the pressure drop across the blockage itself will be equal to the pressure drop across the inlet and outlet manifolds. Such a pressure differential will be sufficient to clear all of the channels of reaction product and condensed humidity.
The above described and many other features and attendant advantages of the present inventions will become apparent as the inventions become better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.